1. Field of the Invention
This invention generally relates to electronic visual display devices and, more particularly, to a color-tunable plasmonic display device that relies upon both modulated and non-modulated refractive index layers.
2. Description of the Related Art
Reflective display or color-tunable device technology is attractive primarily because it consumes substantially less power than liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays. A typical LCD used in a laptop or cellular phone requires internal (backlight) illumination to render a color image. In most operating conditions the internal illumination that is required by these displays is in constant competition with the ambient light of the surrounding environment (e.g., sunlight or indoor overhead lighting). Thus, the available light energy provided by these surroundings is wasted, and in fact, the operation of these displays requires additional power to overcome this ambient light. In contrast, reflective display technology makes good use of the ambient light and consumes substantially less power.
A number of different reflective display technologies have been developed, such as electrophoretic, electrowetting, electrochromic displays, and interference-based MEMS display. These display technologies all have disadvantages or challenges that must be overcome to obtain greater commercial success. Many existing technologies rely upon phenomena that are intrinsically slow. For example, electrophoretic or electrochemical techniques typically require particles to drift or diffuse through liquids over distances that create a slow response. Some other technologies require high power to operate at video rates. For example, many reflective displays must switch a large volume of material or chromophores from one state to another to produce an adequate change in the optical properties of a pixel. At video switching rates, currents on the order of hundreds of mA/cm2 are necessary if a unit charge must be delivered to each dye molecule to affect the change. Therefore, display techniques that rely on reactions to switch dye molecules demand unacceptably high currents for displaying video. The same holds true for electrochromic displays.
A second challenge for reflective displays is the achievement of high quality color. In particular, most reflective display technologies can only produce binary color (color/black) from one material set. Because of this, at least three sub-pixels using different material sets must be used when employing a side-by-side sub-pixel architecture with fixed colors. This limits the maximum reflected light for some colors to about ⅓, so that the pixel's of this type cannot produce saturated colors with a good contrast.
Finally, some reflective displays face reliability problem over a long lifetime. In particular, to sustain video rate operation for a few years requires at least billions of reversible changes in optical properties. Achieving the desired number of cycles is particularly difficult in reflective displays using techniques based on chemical reactions, techniques that involve mixing and separation of particles, or MEMS technology that involves repeated mechanic wear or electric stress.
It would be advantageous if a low power color display existed that overcame all of the above-mentioned problems.